1. Field of the Invention
The present invention relates to a hybrid vehicle having installed therein a motor powered by a battery and an engine using gasoline or gas oil as fuel, and more specifically, to an engine control system for the hybrid vehicle, wherein the battery is rechargeable using the engine.
2. Description of the Prior Art
As compared with the general vehicle having an engine as a driving source, the so-called electric vehicle having a motor as a driving source has advantages of no air pollution, less noise pollution and excellent response in acceleration and deceleration, while having disadvantages of short traveling distance due to the capacity limit of a battery and less economical efficiency due to the high energy unit cost per running distance. Under these circumstances, in order to compensate for the disadvantages of the electric vehicle and to make the best use thereof, various hybrid vehicles having a motor and an engine have been proposed.
There have been available the series hybrid vehicles (SHV) in which only the motor is used as a driving source of the vehicle and the engine is used as a driving source of a generator for charging the battery, and the parallel hybrid vehicles (PHV) in which the motor and the engine are selectively used as driving sources of the vehicle depending on the running condition or the like. The parallel hybrid vehicles include a type in which the motor has a function of a generator for charging the battery.
In charging the battery of the hybrid vehicle, the engine operation is controlled so as to prevent a battery residual amount from becoming less than a preset value. This prevents over charging or over discharging of the battery so as to avoid reduction of the battery life, and further ensures that the battery residual amount is constantly held within the proper range so as to drive the motor reliably.
Further, for improving the exhaust emission and the fuel consumption of the engine, it is necessary that the engine be operated within the optimum emission/fuel consumption range. Japanese First (unexamined) Patent Publication No. 6-245320 discloses a technique for controlling the output of the engine within the optimum emission/fuel consumption range depending on the mean value of the output power of the battery.
However, if the engine output is simply controlled depending on the mean value of the battery output power as in the disclosed technique, it has been found that a charging loss is likely to increase due to the battery charge. Further, when the hybrid vehicle runs on a road having repetitive upward and downward slopes, such as a road in the mountains, it is possible for large current to be drawn from the battery or for the battery to be charged with such as large current. This causes the battery to be subjected to an excessive burden and shortens the life of the battery.